Mammalian H-, K-, and N-Ras proteins, encoded by H-, K-, and N-ras proto-oncogenes, respectively, are 21 kD GTP-binding proteins which possess intrinsic GTPase activity and play a fundamental role in cell proliferation and differentiation (G. L. Bolton, J. S. Sebolt-Leopold, and J. C. Hodges, Annu. Rep. Med. Chem., 1994, 29, 165; R. J. A. Grand in “New Molecular Targets in Cancer Chemotherapy” J. D. Kerr, and P. Workman, Eds., CRC Press, Boca Raton, Fla., 1994, p. 97). Specific mutations in the ras gene impair GTPase activity of Ras, leading to uninterrupted growth signals and to the transformation of normal cells into malignant phenotypes. Mutant ras oncogenes are found in approximately 25% of all human cancers, including 90% of pancreatic, 50% of colon, and 50% of thyroid tumors (J. L. Bos, Cancer Res., 1989, 49, 4682). It has been shown that normal cells transfected with mutant ras gene become cancerous and that unfarnesylated, cytosolic mutant Ras protein does not anchor in cell membranes and cannot induce this transformation (J. F. Hancock, H. Paterson, and C. J. Marshall, Cell, 1990, 63, 133). Posttranslational modification and plasma membrane association of mutant Ras is essential for this transforming activity. The first and required step in the processing of Ras is farnesylation at the cysteine residue of its carboxyl terminal motif, CAAX (C=Cys-186, A=aliphatic amino acid, X=usually methionine, serine or glutamine). Since its identification, the enzyme farnesyl-protein transferase (FPTase) that catalyzes this first processing step has emerged as a promising target for therapeutic intervention (H.-W. Park, S. R. Boduluri, J. F. Moomaw, P. J. Casey, and L. S. Beese, Science, 1997, 275, 1800; P. J. Casey, P. A. Solski, C. J. Der, and J. E. Buss, Proc. Natl. Acad. Sci. U.S.A., 1989, 86, 8323; S. Ayral-Kaloustian and J. S. Skotnicki, Annu. Rep. Med. Chem., 1996, 31, 165, and references therein). Major milestones have been achieved with small molecules, such as mimics of the tetrapeptide CAAX and analogs of farnesyl pyrophosphate, that show efficacy without toxicity in vitro as well as in mouse models bearing ras-dependent tumors or human xenografts with H-, N-, or K-ras mutations (S. Ayral-Kaloustian and J. S. Skotnicki, Annu. Rep. Med. Chem., 1996, 31, 165, and references therein; T. M. Williams, Exp. Opin. Ther. Patents, 1998, 8, 553, and references therein). Several low-molecular weight compounds that inhibit FPTase have entered Phase I trials in humans (SCH-66336, Pharmaprojects, 1998, No. 5128; R-115777, Pharmaprojects, 1998, No. 5532).
Accordingly, there is still a need for drugs for treating and preventing cancer. In particular, there is a need for drugs which inhibit or treat the growth of tumors expressing an activated Ras oncogene and which include cancers of the pancreas, colon, bladder and thyroid.